1. Field of the Invention
This invention relates to cooling of casing of low pressure turbine case of a gas turbine engine and, more particularly, to such cooling by flowing cooling air between shrouds and the case.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine (LPT). The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft to the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aftwardly and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Engine frames are used to support and carry the bearings which, in turn, rotatably support the rotors. Conventional turbofan engines have a fan frame, a turbine center frame, and an aft turbine frame.
The turbine center frame typically has an external casing and an internal hub which are attached to each other through a plurality of multiple radially extending struts. A flowpath frame liner provides a flowpath that guides and directs hot engine gases through the frame and is not intended to carry any structural loads. Cooling air may be introduced into an annular chamber between the external casing and a radially outer flowpath liner of the flowpath frame liner, such as in the GE90. The flowpath frame liner protects the struts and rest of the frame from the hot gases passing through the frame.
Downstream of the turbine center frame is the low pressure turbine. Hot flowpath gases ingested into cavities between the casing and outer flowpath components could transfer heat into the casing by convection. The heat increases the metal temperatures of the casing and in turn reduces the useful life of the casing materials due to low cycle fatigue. The time-dependent properties of the casing material become limiting and unacceptable permanent casing deformations occur that adversely affect interstage turbine clearances, thereby reducing component service life of the casing.
Cooling by way of purge air is provided to annular cavities between the low pressure turbine casing, which for the GE90 is a single piece ring extending across six low pressure stages, and alternating blade shroud segments and low pressure turbine nozzle band segments from which are radially inwardly suspended turbine vane airfoils. Purge air 98 from a turbine center frame 100 of the GE90 engine illustrated in FIGS. 1 and 2 travels through a flow circuit into a small first stage stator cavity 112 and is bounded by an aft 100 rail of a turbine center frame case, a low pressure turbine flange 110 of a low pressure turbine casing 111, and a trailing edge 114 of a first stage stator flowpath outer band 116. Flow passages 118 at a forward lip 120 of a first stage low pressure turbine shroud 122 permits purge air flow to enter a first cavity 124 between the low pressure turbine casing 111 and above the first stage shroud 122. Leakage paths 128 at an aft end 130 of the first cavity 124 and shroud allow the purge air to exit the first cavity. The purge air circuit produces a small reduction in the low pressure turbine casing 111 and low pressure turbine stage one shroud 122 metal temperatures. The ability to purge cooling air from the first cavity 124 above the shroud controls the amount of flowpath gas that can enter the first cavity. The purge or cooling air flow reduces the convection heating of the LPT casing shell. The exiting of this cooling air reduces the heat transfer from the shroud to the LPT Casing by convection and conduction.
Therefore, it would be very beneficial to be able to improve the amount and control of purge air flow in the cavities above shrouds and turbine nozzle bands in the low pressure turbine. It has been found to be particularly useful to cool the first two of these cavities in order to cool the shell of the low pressure casing.